Why does my cylinder turns into a plane in cloth animation
So, it is quite simple, you dont tell blender what is the physic of your rope. A cynlindre with cloth simulation is like an empty cylindre made of fabric.

If you want to create a rope simulation, you will need to apply the simulation on a line and not a cylindre, then draw a cylindre around the line.

I found this video that could help you to achieve what you want.